Does it have to be wall mounted? I just got a semi-concealed one that is brown coloured and sits in the ground, reasonably discreetly. You can get a brown coloured wall mounted electric one for definite so I'd think you'd get a gas one too. Alternately, box over in timber? Tromp lieu?
We moved our meter a few years ago, and looked at the available options, viz: surface mount box (v. ugly), recessed-in-wall box (slightly less ugly), semi-buried box (less conspicuous) or internal.
We rejected all the external options on aesthetic grounds and had the meter mounted intenally. In the fullness of time (how Mrs B loves that phrase) it will be incorporated into a kitchen cupboard.
I don't think transco will install the meter in a non-standard box.
When we were looking at our electic meter cupboard, I didn't want it on the outside wall. We tried to get permission to hide it behind a full height sash window but they wouldn't allow it. Think it was the glass element but it could have been the sliding. So we ended up leaving the top half of the sash and putting doors on the bottom half. Might change it later when they have forgotton about it.
We didn't like the look of the standard boxes either, but after discussions with CO, we were able to compromise and "hide" both of the the standard boxes by burying them behind what look like traditional cellar/ coal chute type doors. We are not able to lock the doors, as gas and electric companies need access - but they are a great deal better than the usual white boxes! Of course you need to have a wall that is thisck enough to take out enough brick/stone to bury the boxes so that the doors will sit flush withthe outside wall. Can post photo's if it would help.